This invention relates generally to solid fuel-burning stoves and more particularly to stoves designed to minimize the emission of smoke from burning fuel into the atmosphere. The term "stove" as used herein is intended to define an enclosed apparatus that burns fuel to provide heat. The term thus encompasses fireplace inserts, boilers, basement furnaces, or the like, as well as the common free-standing stove.
Solid fuel-burning stoves, particularly wood stoves, have become popular in recent years as an inexpensive alternative for home heating. Until this surge of popularity, stoves were not a significant pollution source and their emissions were unregulated. But that has rapidly changed with the increased burning of wood and coal. Stoves have now become a major contributor to air pollution in urban areas. Present wood stoves release large amounts of combustion products such as particulate matter and smoke into the atmosphere.
In recognition of this growing problem, many jurisdictions have passed laws limiting the particulate emission from solid fuel-burning stoves. Missoula, Mont., for example, now monitors and limits particulate emissions from individual stoves because of the pollution they cause during the winter months. Oregon also has recently established emission standards for stoves and fireplace inserts effective in 1986 which specify a maximum number of grams of smoke emitted per hour of use.
Few of the stoves presently on the market can meet the existing and proposed regulations. In response to the Oregon standards, for example, most manufacturers have said that it would be difficult or impossible to meet them without expensive new technology such as a catalytic combustor, which acts as an afterburner to burn combustion products and absorb unburned particles. However, the addition of a catalytic combustor to a conventional stove dramatically increases its cost. The catalytic combustor itself will add $80-$100 to the cost of a new stove. Moreover, for the combustor to work effectively, the catalyst must be replaced regularly at an additional expense.
An alternative to the use of catalytic combustors is the use of secondary combustion chambers which subject the suspended particulate matter and combustible gases to continued or repeated combustion following primary burning of the fuel in a primary combustion chamber. Heretofore, several patents in the prior art have disclosed stoves which use a series of separate combustion chambers to burn fuel more efficiently and thus discharge less particulate matter from the stove. Many of these are primarily designed to achieve maximum burning efficiency. For example, my patents for a free-standing stove Ser. No. 06/403,484, recently allowed, and U.S. Pat. Nos. 4,359,040 and 4,426,992, disclose stoves that have a number of conduits and combustion chambers. Although these stoves effectively reduce particulate emission, their elaborate designs make them relatively expensive.
A simpler and less expensive design is disclosed in U.S. Pat. No. 4,411,181 to Canney. The patent describes a stove having a lower combustion chamber in which the wood is placed and a heating chamber above. Air is supplied beneath the wood for initial combustion. The rising combustion products encounter and flow along the bottom of a flat tube separating the two chambers and through a vertical opening into the heating chamber. Secondary outside air is drawn through the tube, heated, and mixed with the combustion products in the heating chamber to ignite them.
A similar design appears in U.S. Pat. No. 4,154,212 to Wilkinson. The stove has a combustion chamber or firebox for initially burning the fuel and a connected secondary heating chamber immediately above the firebox separated by a common wall. As illustrated in the figures, the combustion products produced within the lower combustion chamber rise directly through several spaced-apart openings in the common wall into the heating chamber where they are heated by the common wall and are immediately swept out of the chamber through the outlet.
Although these last-mentioned dual-chamber stoves offer an improvement over single-chamber stoves in more completely burning the fuel therein, they are lacking in several respects. First, the combustion products are drawn too quickly out of the initial combustion chamber, before initial combustion is completed. Second, too little combustion actually occurs in the second chamber. The relatively large second chamber cannot be sufficiently heated to ignite combustion products far from the common wall. Moreover, the openness of the chamber permits the combustion products to be drawn rapidly through it before they can be mixed with fresh air and re-ignited.
Other multichamber stoves partially overcome these drawbacks, but their complex designs increase their cost, giving them little advantage over catalytic combustors. Furthermore, none of these stoves has demonstrated that they can meet the stringent emission standards established by several governmental air pollution authorities.
It is therefore a primary object of the present invention to provide a solid fuel-burning stove which overcomes the aforementioned drawbacks and limitations of the prior art.
It is a further object of the invention to provide an inexpensive stove of simple design which meets proposed and existing emission standards without the use of a catalytic combustor.
It is yet another object to provide a low emission stove of great versatility in being adaptable for free-standing use, as a fireplace insert or with heat exchanger jackets for mobile home use.